Pontus Jägemalm

Phases, phase transitions and confinement effects in a series of antiferroelectric liquid crystals

Using a variety of optical and electro-optical techniques, as well as dielectric spectroscopy, we have investigated three homologues in the chiral liquid crystal series nF1M7, where n denotes the length of the unbranched terminal chain. The main focus of the study is the series of smectic C subphases, i.e. SmC* α, SmC* 1/3 and SmC* ¼. During switching in the SmC* α phase, a peculiar redirection of the plane of biaxiality, distinguishing this phase from SmA* and SmC*, was observed. We present a simple explanation for this behaviour which correlates well with the clock model description of the SmC* α phase. We found a zero mesoscopic polarization for the SmC* ¼ phase and a non-zero mesoscopic polarization for SmC* 1/3, observations which are consistent with a distorted clock model. The dielectric spectroscopy investigations, performed at several different cell gaps, clearly show that the dielectric response in these materials is easily dominated by surface-induced structures if the cell gap is reduced, thus reflecting that the bulk thermodynamic phase exists in very thick cells only.

Opto-thermal reorientation of nematics with two-fold degenerate alignment

The preferred direction of alignment of the liquid crystal molecules in nematics with two-fold degenerate alignment can be affected substantially by changing the temperature or by applying an electric field. As a result, an almost in-plane switching of the molecules occurs. Here, we report an opto-thermal reorientation effect in a nematic with two-fold degenerate alignment due to a local heating of the liquid crystal by a high power laser beam. The mechanism of this phenomenon is discussed. The opto-thermal reorientation of the molecules makes it possible to visualize the temperature distribution in the illuminated cell and some applications can be foreseen.

Optic, electrooptic and dielectric properties of novel antiferroelectric liquid crystal compounds

Abstract The optic, electrooptic and dielectric spectroscopy characterisation of novel four-ring thiophene-based chiral esters comprising suitably 5, 5′-disubstituted 2, 2′-bithiophene (antiferro- electrics) is reported. The two compounds exhibit interesting hysterises in their phase sequences, and we show that even whole phases (e.g. their relatively wide ferri-phases) may be suppressed by the surface interactions in thin cells.

A New Application of Ferroelectric Liquid Crystals: Fast Electrooptic Helmet Visors for Pulsed Welding Applications

The use of automatically operating shutters for welding is since long a recognized application of liquid crystals with an important market. With the advent and success of the new pulsed welding technique the conventional welding helmets based on nematic liquid crystals are facing difficulties because the nematics are not responding fast enough to stroboscopically follow the changing intensity of the arc during pulsed welding. Ferroelectric liquid crystals (FLCs) here offer new possibilities. Our new FLC-based visor, with a switching time of about 200 microseconds, blocks the high light intensity during the sparks and opens the visor between pulse sparks. This new type of FLC visor thus significantly increases the visibility of the object under work compared to traditional nematic liquid crystal helmet visors, where the relaxation (several milliseconds) is too slow to open the visor between pulses.

Electrically Deformed Short Pitch Chiral Smectic Liquid Crystals

Abstract Short pitch chiral smectic C liquid crystals distorted by a transverse electric field are considered. Fully analytic and very simple expressions are given and discussed for the distorted structure and its optical properties, allowing to calculate the field induced rotation of the effective optic axis.

Critical tilt angle for the optical properties of short pitch chiral smectic liquid crystals

Abstract The optical properties of short pitch chiral smectic C liquid crystals change drastically around a given critical tilt angle, θc. At this angle, the material behaves as an optically isotropic liquid but with a strong optical activity for light propagating orthogonally to the helix axis. It is shown that the polarization properties of the reflected and transmitted beams undergo large changes even for small variations of the tilt angle around the critical one. Some possible applications of the investigated effect are briefly discussed.

Safe and Ergonomic Welding Masks by Fast Liquid Crystals

Recent developments in welding technology, in particular the high intensity pulsed welding arc, have sharpened the requirements on speed and adjustable attenuation for safe and ergonomic operations. With this advent the conventional welding helmets based on nematic liquid crystals are facing difficulties because the nematics are not responding fast enough to stroboscopically follow the changing intensity of the arc during pulsed welding. We propose here a new generation of welding shutters based on smectic (ferroelectric or antiferroelectric) liquid crystals which are fast enough to be synchronized to follow pulsed arc regimes and hence permitting a better visibility of the object under work. The two varieties of smectic materials used are polymer-stabilized in order to make the device shock-proof, to prevent the formation of specific defects such as stripes and chevrons, and to reduce the back-relaxation time in the antiferroelectric case.

Optical activity of small pitch chiral smectic C liquid crystals

Abstract Recently it has been shown that chiral smectics C liquid crystals with pitch shorter than the light wavelength are optically active with maximum rotatory power for light propagating orthogonally to the helix axis. We report the first experimental evidence of this fact.